Some wireless telemetry systems (e.g., burglar alarms, fire alarms, power utility meters, leak detectors, environmental monitoring, etc.) comprise many transmitters that periodically or sporadically transmit messages to one or more receivers. In these systems, the transmitters are located at different places and transmit messages that are indicative of the status of monitoring sensors to a receiver that collects the data from all of the sensors. Normally, the transmitters transmit messages that are as short as feasible and with the interval between the transmissions as long as feasible. This is advantageous for two reasons. First, it minimizes the average current drain in the transmitters, which are typically battery operated. Second, short and infrequent transmissions lower the probability that the data is lost due to collisions that occur when two or more transmitters transmit at the same time. However, if an alarm occurs, the associated transmitter transmits immediately in order to convey the alarm message with little delay.
Typically, such systems transmit data at a single frequency, and thus are susceptible to interference and signal loss due to phenomena known as “multipath fading.” Consequently, the reliability of such systems is compromised or, conversely, the transmitted power has to be increased to overcome the fading, which results in larger power drain and shorter battery life. Besides, there usually are regulatory limits that restrict such transmitter power and thus limit the possible compensation by sheer increase of power. Since the multipath effect is highly sensitive to the frequency of the transmitted carrier, a system using multiple frequencies (e.g., a frequency hopping spread spectrum system, etc.) has a potential to eliminate these drawbacks. However, frequency hopping systems require a long acquisition time and they are typically used in two way communication applications in which all the devices are continuously synchronizing with one master device or with each other using a variety of synchronization methods as shown in some references. In other cases, to ease the synchronization problem, there are employed receivers that can simultaneously receive signals at many frequencies by making the receiver broadband or by using several receivers at the same time. Generally, those solutions suffer from performance degradation or high cost or both, which makes them undesirable for low cost applications that require high reliability such as security systems and some telemetry systems.
A serious problem that must be addressed in battery operated systems is to shorten the transmission time as much as possible by making the message preamble as short as possible in order to conserve the battery power. Therefore, the synchronization of the receiver with the transmitters is a difficult task. This problem is exacerbated in some systems such as security alarms that require some messages to be conveyed to the system without waiting for the scheduled transmission time. A related problem, in battery operated systems, is limitation of the transmitted power to conserve the battery power.
Many telemetry systems need to be protected from tampering with telemetry data. For example, in a security system, an intruder may attempt to gain access by spoofing the system with false transmissions. It is therefore important to provide for authentication of the transmissions. Similarly, some telemetry systems need to protect data from eves dropping, thus requiring encryption to protect the transmitted data.